Add egg yolks, flour, sugar, cocoa and vanilla to a small pot and mix. Add milk and cream and whisk over a medium heat until mixture has thickened to a pudding like consistency. Immediately remove from heat. Place into a covered container and place into the refrigerator until fully cooked or overnight.
Craquelin Top
Melt butter and add to a bowl with brown sugar, cocoa and flour. Mix. Place mixture between 2 pieces of parchment paper and roll until flattened. Place on a cookie sheet and put into the freezer.
Choux
Place water, sugar and butter in a saucepan. Heat until butter is melted. Turn off the heat and add flour. Mix. Add mixture to a standing mixer and mix for 5 minutes to cool it. Add eggs one at a time until blended.Pour mixture into a pastry bag with a large round tip. Pipe 1 & 1/2- 2 inch rounds on a parchment lined baking sheet.Remove craquelin from the freezer. Peel off the top layer of parchment paper. With a round cookie cutter, the same size of the piped choux, cut enough circles to cover each of the piped choux and place them on top.Bake in a pre-heated 350 degree oven for 20 minutes. Open oven to let the steam out, then bake for an additional 15 minutes or until browned. Cool on a wire rack.
Meringue
In the metal stand mixer bowl set over a saucepan of simmering water, whisk the egg whites with the sugar until the whites are warm and the sugar is dissolved, about 3 minutes until hot to touch. Take the bowl off of the water and attach it to the stand mixer fitted with the whisk. Add the vanilla and cream of tartar and beat at medium speed until firm. Increase the speed to high and beat the meringue until stiff and glossy, 5 to 7 minutes.
Assembly
Cut the tops off of each pastry. Add about 1-2 teaspoons of chocolate pastry cream to the bottoms. Add marshmallow meringue to a pastry bag with a star tip. Pipe cream on top. Use a hand torch to toast the meringues. Top with a chocolate square, then place other half of pastry on top.
